A chemical name of ramosetron is (−)-(R)-5-[(1-methyl-1H-indol-3-yl)carbonyl]carbonyl-4,5,6,7-tetra-hydro-1H-benzimidazole. A series of tetrahydrobenzimidazole derivatives including said ramosetron and pharmaceutically acceptable salts thereof are reported as a useful drug compound having an excellent antagonistic action against a serotonin (5-HT3) receptor and suppressing diseases of digestive tract induced by administration of anticancer agents, such as nausea and vomiting (see Patent Document 1), and in particular, a hydrochloride of ramosetron is already marketed (hereinafter, the marketed drug compound will be referred to as “ramosetron hydrochloride”). It is known that the ramosetron hydrochloride exhibits an excellent pharmacological effect against adults upon its oral administration of 0.1 mg once a day, and it is on sale as a trade name of “Nasea OD Tablets 0.1 mg” from Yamanouchi Pharmaceutical Co., Ltd.
Also, in view of the fact that serotonin receptor antagonists irritate a serotonin (5-HT3) receptor and increase liberation of acetylcholine, they are expected to be applicable as a therapeutic agent of irritable bowel syndrome (IBS). However, a few of serotonin receptor antagonists are clinically confirmed to have a therapeutic effect against patients of irritable bowel syndrome, and with respect to the ramosetron hydrochloride, its effectiveness has not been reported yet.
The present inventors obtained an idea that an effective amount of ramosetron hydrochloride for therapy against irritable bowel syndrome may possibly be far low as compared with 0.1 mg as the administration amount that is currently employed for a depressor of diseases of digestive tract induced by the administration of anticancer agents.
However, in general, in the case of formulating a drug compound, the lower the content becomes, the more likely the drug compound suffers from a mutual action with drug additives, and therefore, the drug compound is worried about a lowering of its stability.
The 0.1 mg tablet of ramosetron hydrochloride as a product employs a packaging state into which a desiccant is incorporated. Accordingly, this product was a pharmaceutically stable formulation and did not have a problem as goods on the market. However, it is thought that in low-content formulations, a stabilizing effect is insufficient only by the desiccant.
[Patent Document 1] European Patent No. 381,422